Observed time difference of arrival angle of arrival discriminator

ABSTRACT

A method, user equipment (UE) and location server for estimating position of a UE based on observed angles of arrival. According to one embodiment, angles of arrival of signals from a plurality of base stations are received by a UE are observed by scanning for position reference signals (PRS) by adjusting a phase difference between antennas to cause a null of a beam of the UE to be incremented through an angular sector. For each of a plurality of base stations, an angle of arrival at which the null is steered when a PRS is suppressed by the null and a reference signal time difference, RSTD, are determined. Each angle of arrival and corresponding RSTD is transmitted to a location server which estimates UE position based on the observed angles of arrival. Further, the location server may instruct the UE to suppress a non-line-of-sight PRS signal.

TECHNICAL FIELD

The present disclosure relates to wireless communication systems, andmore particularly to determining angles of arrivals of positionreference signals for determining a wireless user equipment location.

BACKGROUND

When a user equipment (UE), such as a cellular phone, is used to dial a911 emergency call center, government regulations call for an ability todetermine the location of the UE so that the location may automaticallybe sent to the 911 emergency call center. Many UEs are equipped with aglobal positioning system (GPS) receiver that can accurately determinethe UE location. However, if the link to GPS satellites is weak or theUE does not have GPS capability, then another method of positiondetermination is called for.

One such method is a technique called observed time difference ofarrival (OTDOA). This technique operates by transmitting a specialphysical channel called a position reference signal (PRS) to the UE from3 or more base stations such as evolved node Bs (eNBs) in a long termevolution (LTE) communication network. The PRS includes precise timinginformation. The UE observes the relative arrival times of the signalsfrom nearby eNBs and reports the reference signal time difference (RSTD)observations to a particular serving eNB to which the UE is activelyattached. The serving eNB conveys the report of observations to alocation server which computes the position estimate of the UE usingtriangulation. The OTDOA mechanism relics upon accurate knowledge of theantenna locations of the cNBs, an eNB time reference relative to theother eNBs, and the observed time of arrival of a PRS signal at the UE.An error in any of these parameters degrades the position estimate ofthe UE.

The time of arrival (TOA) of the PRS is presumed to be received from aline of sight (LOS) direction, i.e., a direct path from an antenna of aneNB. Depending on the propagation environment, the LOS path may beattenuated due to an obstruction, such that a relatively stronger signalfrom the antenna may arrive from the eNB at the UE via a path thatincludes a reflection. This signal may drown out the LOS signal and mayconvey inaccurate TOA information.

The accuracy of the estimate of the location of the UE by the locationserver may be adversely affected by non-LOS propagation of the PRSreceived at the UE such as may result from reflected multipath signals.

SUMMARY

Methods and systems for determining a UE location based on signal angleof arrival observations are disclosed. According to one aspect, there isprovided a location server for a wireless communication system. Thelocation server includes a memory and a processor. The memory isconfigured to store reference signal time differences (RSTD), receivedfrom a user equipment (UE). Each RSTD is based on a position referencesignal (PRS) received by the UE from a different one of a plurality ofbase stations. The memory is also configured to store observed angles ofarrival received from the UE, each observed angle of arrivalcorresponding to a different one of the RSTDs. The processor is incommunication with the memory and is configured to determine a firstestimated UE location based on the RSTDs and the known locations of theplurality of base stations. For each RSTD, the processor is configuredto determine an estimated angle of arrival based on the first estimatedUE location and known locations of the base stations. The processor isalso configured to compare an observed angle of arrival with theestimated angle of arrival corresponding to the RSTD, and to weight theRSTD based on the comparison. The processor may also be configured todetermine a second estimated UE location based on at least some of theweighted RSTDs and known locations of the plurality of base stations.

According to this aspect, in some embodiments, the location server islocated at one of the plurality of base stations. In some embodiments, amagnitude of the weight assigned to an RSTD is inversely proportional toa difference between the estimated angle of arrival and the observedangle of arrival. In some embodiments, a set of three RSTDs with thehighest weights is used to estimate the location of the UE. In someembodiments, the second estimated UE location is based on a leastsquares operation applied to the RSTDs. In some embodiments, theprocessor is further configured to send an instruction to the UE tosuppress a particular one of the PRSs from the plurality of basestations. In some embodiments, the PRS to be suppressed may beassociated with an RSTD that has a weight that is lower than athreshold.

According to another aspect, there is provided a UE capable of observingangles of arrival of a plurality of position reference signals (PRS).The UE includes a plurality of antennas and a phase shifter to adjust arelative phase between at least two of the plurality of antennas. Atransceiver of the UE is configured to receive and process signals fromthe antennas, the signals including at least one position referencesignal. PRS. A memory is configured to store an angle of arrival of eachof the at least one PRS. A processor coupled to the phase shifter, thetransceiver, and the memory is configured, for each of the at least onePRS, to adjust the phase shifter to adjust a phase between antennas tosteer a null in a signal processed by the transceiver until the PRS issuppressed.

According to this aspect, in some embodiments, the processor is furtherconfigured to determine an angle of arrival of the PRS as being theangle to which the null is steered when the PRS is suppressed, and isfurther configured to cause transmission, via the transceiver, of thedetermined angle of arrival to a location server. In some embodiments,the PRS that is suppressed is selected by a location server as beingreceived from a direction other than along a line of sight between theUE and a base station from which the PRS is transmitted. In someembodiments, the processor is further configured to, after suppressingthe indicated PRS from the non-line of sight direction, detect the PRSreceived at a different angle of arrival from the base station fromwhich the suppressed PRS is received. In some embodiments, a sensor isconfigured to enable determination of a spatial orientation of the UE.The sensor establishes a reference plane for determining an angle towhich the null is steered. In some embodiments, the UE determines anglesof arrivals of multiple PRS, each PRS received from a different one of aplurality of base stations.

According to another aspect, there is provided a method of estimating aposition of a user equipment (UE) in a wireless communication system.The method includes receiving from the UE reference signal timedifferences (RSTDs), each RSTD being based on a position referencesignal (PRS) received by the UE from a different base station. Themethod also includes receiving from the UE observed angles of arrival,each observed angle of arrival corresponding to a different one of theRSTDs. A first estimate of the position of the UE is determined based onthe RSTDs and known locations of the base stations corresponding to theRSTDs. For each RSTD, the method includes determining an estimated angleof arrival based on the first estimated position of the UE and the knownlocations of the base stations; comparing an observed angle of arrivalto the estimated angle of arrival; and weighting the corresponding RSTDbased on the comparison. The method also includes performing a secondestimate of the position of the UE based on the weighted RSTDs and theknown locations of the base stations corresponding to the RSTDs.

According to this aspect, in some embodiments, the second estimate isperformed based on three weighted RSTDs having highest weights. In someembodiments, the weighted RSTD corresponds to a line-of-sight angle ofarrival. In some embodiments, the method further includes sending aninstruction to the UE to suppress a particular PRS arriving at anobserved angle of arrival based on a determination that the particularPRS corresponds to an observed angle of arrival that differs from itscorresponding estimated angle of arrival by more than a threshold. Insome embodiments, a magnitude of a weight is inversely proportional to adifference between the observed angle of arrival and the estimated angleof arrival corresponding to the RSTD to which the weight is applied.

According to another aspect, there is provided a method of generatingobserved angles of arrival of signals from a plurality of base stationsand received by a user equipment. UE. The method includes scanning forposition reference signals, PRSs, by adjusting a phase differencebetween antennas to cause a null of a beam of the UE to be incrementedthrough an angular sector. The method also includes determining, foreach of the plurality of base stations: an angle of arrival at which thenull is steered when a PRS is suppressed by the null; and a referencesignal time difference, RSTD. The method also includes transmitting eachangle of arrival and corresponding RSTD to a location server.

According to this aspect, in some embodiments, the method includesdetermining a spatial orientation of the UE to determine a referenceplane for the scanning. In some embodiments, the method includesreceiving from the location server an indication of a particular one ofthe PRSs to be suppressed, the PRS to be suppressed being determined tobe received from a non-line-of-sight direction. In some embodiments, theindication includes a target angle of arrival at which the PRS to besuppressed is received, and the method further comprises steering a nullto the target angle of arrival. In some embodiments, for a particularbase station and PRS, the method includes scanning the null to determineif more than one minima of the PRS is encountered, more than one minimaindicating that the PRS is received via multiple paths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a system for determining location of a userequipment according to principles of the present disclosure;

FIG. 2 is a block diagram of a communication system having a basestation, a user equipment and a location server arranged to estimate UEposition in accordance with principles of the present disclosure;

FIG. 3 is a flowchart of a process for determining and reporting anglesof arrival of position reference signals (PRS) at a UE;

FIG. 4 is a flowchart of a process for calculating a first estimate ofUE position based on angles of arrival determined by the UE andtransmitted to the location server;

FIG. 5 is a flowchart for calculating a second estimate of UE positionbased on weighted RSTDs;

FIG. 6 is a flowchart of a process for determining when to discard anRSTD and instruct a UE to suppress the PRS corresponding to thediscarded RSTD; and

FIG. 7 is a flowchart of a process for suppressing a PRS received from anon-LOS angle of arrival.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail example embodiments that are in accordancewith the present disclosure, it is noted that the embodiments resideprimarily in combinations of apparatus components and processing stepsrelated to determining position of a UE. Accordingly, the system andmethod components have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments of thepresent disclosure so as not to obscure the disclosure with details thatwill be readily apparent to those of ordinary skill in the art havingthe benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements

In embodiments discussed herein, AOA data associated with a receivedposition reference signal (PRS) is collected at the user equipment (UE)and sent to a location server along with corresponding reference signaltime difference (RSTD) observations. The location server uses thisinformation along with knowledge of radiating antenna coordinates ofantennas of the neighboring base stations to establish an estimatedangle of arrival. AOAs at the UE can be established using the phaserelationship between two or more receive antenna elements of the UE. Insome embodiments, the location server uses this estimate to establish alikelihood that one of many PRS arrive by way of a line of sight (LOS)propagation from a given radiating antenna. Based on this likelihood, aweight is applied to the RSTD of each set of observed time difference ofarrival (OTDOA) data. Due to a finite dynamic range of the UE receiverin detecting the PRS, detection of a weak line of sight PRS that isimpaired by some attenuating obstruction may be degraded in the presenceof a strong non-line of sight PRS originating from the same radiatingantenna. If this is determined to be the case, a null can be applied inthe direction of the stronger non-line of sight signal in order toimprove reception of the line of sight signal which can be identified byits faster arrival time.

A null occurs when the output of individual antenna elements are summedwith a specific phase offset applied to each element. Because the nullresponse can be made quite sharp, the null response has good qualitiesfor establishing an AOA. In particular, the angle of arrival of a givenPRS may be established with good accuracy at a UE with a limited numberof antenna elements by phasing antennas at the UE to null out the PRSsignal. The angle of arrival of the PRS is determined to be the angle atwhich the null is steered when the received PRS signal strength is aminimum. Calculation of the angle of arrival may be made relative to aknown planar orientation of the UE's antenna elements. Accelerometersused as orientation sensors are commonly installed in UE devices and maybe used to establish the planar orientation of the UE's antennaelements.

Referring now to the drawing figures in which like reference designatorsdenote like elements, there is shown in FIG. 1 a diagram of aconfiguration 10 of neighboring base stations 12 a, 12 b, 12 c and 12 d,referred to collectively herein as base stations 12, or, in long termevolution (LTE) networks, eNBs 12. Each base station 12 transmits a PRSto a UE 14. In the example of FIG. 1, the base stations 12 b, 12 c and12 d transmit to the UE using a line of sight (LOS), i.e., a direct pathfrom the base station antenna to the UE. As shown in FIG. 1, the signalfrom the base station 12 a arrives from a line of sight that has anobstacle 16 a that attenuates the transmitted signal. The signal fromthe base station 12 a also arrives from a non-LOS direction via areflection from an obstacle 16 b. Thus, in the example of FIG. 1, it ispossible that a signal from a non-LOS direction may be stronger than thesame signal from a LOS direction.

A location server 18, which may be part of a base station 12, may be indirect communication with the UE 14 or may receive OTDOA signalmeasurement information from the UE 14 indirectly via a base station 12.In some example embodiments, the location server 18 communicates adetermined location of the UE via communication network 20 to a 911 callcenter 22. In operation, the UE 14 observes PRS from the various basestations and determines their angles of arrival. The UE also determines,for each PRS, an RSTD based on a time of flight of the PRS. The anglesof arrival and the corresponding RSTDs obtained from the PRS are sent tothe location server 18.

The location server 18 determines the location of the UE 14 based on theRSTDs and the known locations of the base stations 12. Further, for eachRSTD, the location server 18 compares an estimated angle of arrival,computed based on known locations of the base stations 12, to an angleof arrival observed at the UE 14, to determine a weight to be applied toan RSTD. A greater weight may be assigned to an RSTD when the estimatedangle of arrival is close to the observed angle of arrival, and a lesserweight may be assigned to the RSTD when the estimated angle of arrivaldiffers substantially from the observed angle of arrival. Then, in someexample embodiments, the location server 18 computes a more accurateestimation of the position of the UE 14 using some or all of theweighted RSTDs. This is described in more detail below with reference tothe flowcharts of FIGS. 4A and 4B.

FIG. 2 is a more detailed block diagram of a communication system 10having the base station 12, the user equipment 14 and the locationserver 18. Note that the location server 18 may, in some embodiments, belocated at the base station 12, and in some embodiments, may communicateonly indirectly with the UE 14 via a base station 12. The UE 14 has atleast two antennas 24 for receiving PRS and for transmitting OTDOAsignal measurement information, which includes RTSD and AOA, and mayalso include LOS quality data. A phase shifter 26 enables shifting ofthe relative phase between signals received by the antennas 24 in orderto sweep a null signal through an angular sector. A transceiver 28 ofthe user equipment 14 transmits the radio frequency (RF) signalscarrying the OTDOA signal measurement information to the location server18, and receives PRS from a plurality of base stations 12.

The UE 14 includes a memory 30 and a processor 32. The memory 30 storesobserved angles of arrival 34. RSTDs 36 and phase values 40 foradjusting the phase shifter 26. The memory may also be configured tostore computer code that, when executed by the processor 32, causes theprocessor 32 to perform UE functions described herein. The processor 32,when executing software, includes functionality to cause null steering42 and to determine observed angles of arrival 44. In particular, theprocessor may perform operations that include adjusting a phase value40, measuring a PRS magnitude, continuing to adjust phase until aminimum in the PRS magnitude occurs, and determining the angle of thenull at which the minimum occurs to be the angle of arrival for theparticular PRS.

Further, the processor 32 may cause suppression of a PRS signal receivedat a particular angle, while searching fir a minimum of the PRS signaloccurring at another angle. Where multiple PRS minimums occur, theprocessor 32 may determine the angle of arrival of the PRS 44, having aleast RSTD. Note that the UE 14 may also include a sensor such as anaccelerometer 46 to ascertain an orientation of the UE 14 to ensure thatthe UE is in a particular orientation, such as vertical, for determiningan observed angle of arrival with respect to a reference plane.

In some embodiments, a PRS arriving from a particular angle that isdetermined by the location server 18 to be from a direction other than aline of sight, may be suppressed. The non-LOS signal from the particularangle may be suppressed by steering a null to the particular angle atthe UE 14. In some embodiments, the processor 32 of the UE 14 is furtherconfigured, after suppressing the indicated PRS from the non-line ofsight direction, to detect the PRS received from a different angle ofarrival. Thus, the UE 14 may perform any one or more of at least twomajor functions: (1) determining observed angles of arrival andcorresponding RSTDs to be used by the location server 18 to determine UEposition, and (2) suppressing PRS from non-line of sight directions anddetecting the PRS at another angle of arrival. Suppressing a PRS from anon-LOS angle of arrival of a PRS can improve reception of the PRSarriving at a LOS angle of arrival.

Note that the OTDOA information may be sent to the location server 18directly from the UE 14. Alternatively, the OTDOA information may besent by the UE 14 to the base station 12 from which it is forwarded tothe location server 18. In some embodiments, the location server 18 maybe located at a base station 12. The base station 12 includes at leastone antenna 48, a transceiver 50, a memory 52 and a processor 54. Thetransceiver 50 operates to prepare data to be transmitted and to convertreceived data to baseband for processing by the processor 54. The memory52 stores position reference signals (PRS) 56 and a location 58 of thebase station 12. The memory 52 may also be configured to store computercode that, when executed by a processor 54, causes the processor 54 toperform base station functions described herein. The processor 54, whenexecuting software, generates, via a PRS generator 60, the PRS fortransmission to the UE 14 by the transceiver 50 of the base station 12.

The location server 18 includes at least one antenna 62 and atransceiver 64 to receive OTDOA measurement information from the UE 14.The location server 18 also includes a memory 66 and a processor 68. Thememory 66 stores observed and estimated angles of arrival 70, RSTDs 72,base station locations 74, and estimated UE locations 76. The memory 66may also be configured to execute computer code that, when executed by aprocessor 68, causes the processor 68 to perform location serverfunctions described herein. The processor 68, when executing computercode, functions to perform UE location estimation 78, angle of arrival(AOA) estimation 80, AOA comparison 82, and weight assignment 84.

In operation, the processor 68 of the location server 18 computesestimated UE locations based on RSTDs and known base station locations,and computes estimated UE locations based on the observed angles ofarrival and known base station locations via trilateration, i.e., bydetermining an intersection of lies extending from the base stations atthe observed angles of arrival. The processor 68 may also compute anglesof arrival based on the UE location estimate and known base stationlocations, and compare these computed angles of arrival to angles ofarrival observed at the UE 14. The processor 68 may eliminate RSTDs fromthe computation of UE location when the difference between the observedand estimated AOA corresponding to the RSTD is large, i.e., greater thana predetermined amount. The processor 68 may also assign a weight to anRSTD that depends upon a magnitude of a difference between the estimatedangle of arrival and the observed angle of arrival. Thus, the locationserver 18 may perform any one or more of at least three major functions:(1) comparing an observed angle of arrival to an estimated angle ofarrival to determine a weight to be assigned to a corresponding RSTD,(2) computing an estimated UE position based on the weighted RSTDs and(3) instructing the UE 14 to suppress a particular one of the PRScorresponding to an RSTD determined to be from a non-line-of-sight.

The processor 68 of the location server 18 is in communication with thememory 66 and is configured to determine a first estimated UE locationbased on the RSTDs and the known locations 74 of the plurality of basestations 12. For each RSTD, the processor 68 is configured to determinean estimated angle of arrival 70 based on the first estimated UElocation 76 and known locations of the base stations 74. The processor68 is also configured to compare an observed angle of arrival with theestimated angle of arrival corresponding to the RSTD, and to weight theRSTD based on the comparison. In some embodiments, the processor 68 isalso configured to determine a second estimated UE location 100 based onat least some of the weighted RSTDs and 74 known locations of theplurality of base stations. Thus, improved accuracy in determining theposition of a UE may be achieved in some embodiments by assigning higherweighting to LOS data and lower weighting to non-LOS data. The locationserver 18 may optionally instruct the UE to suppress a PRS determined tobe from a non-line of sight direction FIG. 3 is a flowchart of a processperformed by the UE 14 for determining and reporting angles of arrivalof position reference signals at a UE 14. Not shown in FIG. 3 is anoptional initial step of determining UE orientation. Thus, theorientation sensors 46 of the UE 14 may be used to determine if the UE14 has a specific planar orientation that is acceptable for determiningangles of arrival. For example, the sensors 46 may determine whether theUE 14 is oriented vertically plus or minus 30°. In some embodiments, anull value is reported with the OTDOA signal measurement informationfrom the UE 14 to the location server 18 if a suitable orientation ofthe UE 14 is not found within a pre-determined amount of time, the nullvalue indicating that an angle of arrival has not been detected for aparticular PRS.

If the orientation of the UE 14 is acceptable, then, for each of aplurality of PRSs, the signals received at each of at least two antennas24 of the UE 14 are phased with respect to each other to sweep a null ina beam formed by the antennas 24 through an angular sector such as 360°(block S104). A phase shifter 26 for varying the phase between thesignals may be incremented according to pre-determined increments. Theangle of arrival for a particular PRS is determined as the angle atwhich the PRS minimum occurs. In other words, the angle to which thenull is steered to suppress the PRS signal is the observed angle ofarrival of the PRS signal (block S106). Additionally, the beam null mayoptionally be scanned so as to determine whether more than one minima ofa particular PRS being scanned is encountered, thereby indicating thatthe PRS is received by multiple paths. One of the angles of arrival, forexample, the angle of arrival from a base station to which the UE isattached, may be used as a reference angle against which the otherangles of arrival are measured. Each determined angle of arrival isassociated with an RSTD determined by the UE (block S108). Each observedangle of arrival and corresponding RSTD is transmitted from the UE 14 tothe location server 18 (block S110).

FIG. 4 is a flowchart of a process performed by the location server 18for calculating the estimated position of a UE 14 based on angles ofarrival determined by the UE 14 and transmitted to the location server18. OTDOA signal measurement information is received by the locationserver 18 (block S112). The OTDOA signal measurement informationincludes at least two UE observations for each of a plurality of basestations 12. The at least two UE observations include an RSTD and anobserved angle of arrival. A first estimated UE location is calculatedusing multilateration based on the received RSTDs observed at the UE 14and knowledge of the locations of the eNBs 12 possessed by the locationserver 18 (block S114). An estimated angle of arrival from a particularcNB 12 is calculated based on the estimated UE location and physicallocations of the eNBs 12 (block S116).

FIG. 5 is a flowchart of a process for weighting RSTDs and computing asecond estimate of UE position. For each RSTD, the estimated angle ofarrival is compared to the observed angle of arrival (block S118).

A weight may be assigned to the RSTD based on the difference between theobserved angle of arrival and the estimated angle of arrival (blockS120). A magnitude of a weight assigned to an RSTD may be inverselyproportional to a difference between the estimated angle of arrival andthe observed angle of arrival, so that a larger weight is applied whenthe difference between the estimated AOA and the observed AOA is small,and a smaller weight is applied when the difference is large. After allthe RSTDs have been weighted, a second estimate of the UE may becalculated using the highest weighted RSTDs and the known base stationlocations (block S122).

Thus, if the difference between the observed angle of arrival and theestimated angle of arrival is sufficiently small as compared to athreshold for each of at least three RSTDs, then the UE location may beestimated using multilateration and the weighted RSTDs. RSTDscorresponding to a difference between the observed angle of arrival andan estimated angle of arrival that is not as small as the threshold maybe discarded. When the total number of non-discarded RSTDs, includingthe RSTD of the reference eNB 12, is greater than two, thentrilateration based on the 3 RSTDs having the highest quality may beperformed. Also, more than three RSTDs may be employed in a weightedformula, such as a least squares method, to compute the UE location.This second estimate of UE location is, in some embodiments, provided bythe location server 18 to the 911 call center 22 via the network 20.Note that the embodiments of FIGS. 4 and 5 contemplate the possibilityof using a non-line-of-sight PRS in a least squares calculation ofestimated UE position. For example, some RSTDs that are less than thethreshold may correspond to non-LOS PRS. This may result from a non-LOSpath being very short or from a condition where many non-LOS PRS signalsare received from abase station.

FIG. 6 is a flowchart of a process for making a determination to discardan RSTD and optionally instruct the UE to suppress the corresponding PRSarriving from the corresponding observed angle of arrival. As mentionedabove with reference to FIG. 5, the location server 18 compares, foreach RSTD, an observed angle of arrival to an estimated angle of arrival(block S118). If the difference between the observed angle of arrivaland the estimated angle of arrival exceeds the threshold, thecorresponding RSTD is discarded (block S124). In some embodiments, themethod includes sending an instruction to the UE to suppress aparticular PRS arriving at an observed angle of arrival corresponding tothe discarded RSTD, which is considered to be from a non-line-of-sightdirection (block S126).

FIG. 7 is a flowchart of an example process for suppressing PRS receivedfrom the non-LOS angles of arrival. The UE 14 receives the instructionfrom the location server 18 to suppress a PRS at a target angle ofarrival from the location server (block S128). The PRS to be suppressedis one that is deemed to arrive by a non-LOS direction, based on theobserved angle of arrival differing from the estimated angle of arrivalby an amount exceeding the threshold.

The UE 14 may scan a null in the beam to the target angle of arrival tosuppress the particular PRS (block S130). Suppressing a high powernon-LOS version of the PRS enables better detection of a lower power LOSversion of the PRS that arrives earlier than the suppressed PRS arrivingat the target angle. Thus, a first null may be steered to the angle ofarrival of the high power non-LOS version of the PRS and a second nullmay be swept to determine an angle of arrival for one or more versionsof the PRS signal that has a smaller RSTD value. Once the PRS from thenon-LOS direction is suppressed, the UE 14 may scan its beam to find thePRS arriving at a different angle of arrival (block S132). This angleand a corresponding RSTD may be sent to the location server 18 (blockS134).

Note that in some embodiments, a value may be assigned to anotherparameter called “LOS quality” or “RSTD quality” measured by the UE andincluded within the OTDOA signal measurement information. A large LOSquality rating may be assigned to strongly received PRS and small LOSquality rating may be assigned to weakly received PRS. When thisinformation is received by the location server, the location server mayweight a corresponding RSTD based at least in part on the received LOSquality rating, where signals having a large LOS quality rating areweighted more heavily than signals having lower LOS quality rating.Thus, the LOS quality rating may be based on the PRS signal strength andmay be affected by signal to noise ratio of the PRS, signal to noiseratio of a reference PRS, detected orientation of the UE 14, and thenumber of UE antennas.

Thus, embodiments may provide improved accuracy of position estimates ofa UE by assigning weights to RSTDs measured by the UE based on acomparison of an angle of arrival observed by the UE to an estimatedangle of arrival computed by a location server. Non-LOS PRSs received bythe UE may be nulled to enable better detection of a PRS received from aLOS direction or a stronger PRS received from a non-LOS direction.

The present invention can be realized in hardware, or a combination ofhardware and software. Any kind of computing system, or other apparatusadapted for carrying out the methods described herein, is suited toperform the functions described herein. A typical combination ofhardware and software could be a specialized computer system, having oneor more processing elements and a computer program stored on a storagemedium that, when loaded and executed, controls the computer system suchthat it carries out the methods described herein. The present inventioncan also be embedded in a computer program product, which comprises allthe features enabling the implementation of the methods describedherein, and which, when loaded m a computing system is able to carry outthese methods. Storage medium refers to any volatile or non-volatilestorage device.

Computer program or application in the present context means anyexpression, in any language, code or notation, of a set of instructionsintended to cause a system having an information processing capabilityto perform a particular function either directly or after either or bothof the following a) conversion to another language, code or notation; b)reproduction in a different material form.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope of thefollowing claims.

1-23. (canceled)
 24. A method performed by a user equipment (UE), themethod comprising: scanning for position reference signals (PRSs) from aplurality of base stations, by adjusting a phase difference betweenantennas to cause a null of a beam of an antenna pattern of the UE to beincremented through an angular sector; determining, for each of theplurality of base stations: an observed angle of arrival (AOA) at whichthe null is steered when a PRS is suppressed by the null; and areference signal time difference (RSTD); and transmitting each observedAOA and corresponding RSTD to a location server.
 25. The method of claim24, further comprising determining a spatial orientation of the UE todetermine a reference plane for the scanning.
 26. The method of claim24, further comprising receiving from the location server an indicationof a target PRS to be suppressed, the target PRS being determined to bereceived from a non-line-of-sight direction.
 27. The method of claim 26,wherein the indication includes a target AOA at which the target PRS isreceived, and the method further comprises steering a null to the targetAOA.
 28. The method of claim 27, wherein the target AOA differs, fromthe observed AOA corresponding to the target PRS, by more than athreshold.
 29. The method of claim 24, further comprising: for a firstPRS received from one of the plurality of base stations, scanning thenull to determine if more than one minimum of the first PRS isencountered, the more than one minimum indicating that the first PRS isreceived via multiple paths.
 30. A user equipment (UE), comprising: aplurality of antennas; a phase shifter configured to adjust a relativephase between at least two of the plurality of antennas; a transceiverconfigured to receive and process signals from the plurality ofantennas, the signals including at least one position reference signal(PRS); a memory storing instructions; and a processor configured toexecute the instructions to: scan for PRSs from a plurality of basestations, by controlling the phase shifter to adjust a phase differencebetween antennas to cause a null of a beam of an antenna pattern of theUE to be incremented through an angular sector; determine, for each ofthe plurality of base stations: an observed angle of arrival (AOA) atwhich the null is steered when a PRS is suppressed by the null; and areference signal time difference (RSTD); and transmit each observed AOAand corresponding RSTD to a location server.
 31. The UE of claim 30,wherein the processor is further configured to execute the instructionsto: determine a spatial orientation of the UE to determine a referenceplane for the scanning.
 32. The UE of claim 30, wherein the processor isfurther configured to execute the instructions to: receive from thelocation server an indication of a target PRS to be suppressed, thetarget PRS being determined to be received from a non-line-of-sightdirection.
 33. The UE of claim 32, wherein the indication includes atarget AOA at which the target PRS is received, and the processor isfurther configured to execute the instructions to steer a null to thetarget AOA.
 34. The UE of claim 33, wherein the target AOA differs, fromthe observed AOA corresponding to the target PRS, by more than athreshold.
 35. The UE of claim 30, wherein the processor is furtherconfigured to execute the instructions to: for a first PRS received fromone of the plurality of base stations, scan the null to determine ifmore than one minimum of the first PRS is encountered, the more than oneminimum indicating that the first PRS is received via multiple paths.36. A non-transitory computer-readable medium storing a program codeexecutable by a user equipment (UE), wherein the execution of theprogram code causes the UE to perform operations comprising: scanningfor position reference signals (PRSs) from a plurality of base stations,by adjusting a phase difference between antennas to cause a null of abeam of an antenna pattern of the UE to be incremented through anangular sector; determining, for each of the plurality of base stations:an observed angle of arrival (AOA) at which the null is steered when aPRS is suppressed by the null; and a reference signal time difference(RSTD); and transmitting each observed AOA and corresponding RSTD to alocation server.
 37. The non-transitory computer-readable medium ofclaim 36, wherein the operations further comprise: determining a spatialorientation of the UE to determine a reference plane for the scanning.38. The non-transitory computer-readable medium of claim 36, wherein theoperations further comprise: receiving from the location server anindication of a target PRS to be suppressed, the target PRS beingdetermined to be received from a non-line-of-sight direction.
 39. Thenon-transitory computer-readable medium of claim 38, wherein theindication includes a target AOA at which the target PRS is received,and the operations further comprise steering a null to the target AOA.40. The non-transitory computer-readable medium of claim 39, wherein thetarget AOA differs, from the observed AOA corresponding to the targetPRS, by more than a threshold.
 41. The non-transitory computer-readablemedium of claim 36, wherein the operations further comprise: for a firstPRS received from one of the plurality of base stations, scanning thenull to determine if more than one minimum of the first PRS isencountered, the more than one minimum indicating that the first PRS isreceived via multiple paths.